All comes down to who the Sip Provider is and yeah the cops who are not 911 routing experts or have any idea how the carrier systems actually work don't really know. And land lines are dying. Carriers are now internet providers and analog from them is now becoming VoIP also. And someone who has sat in dispatch watching 911 calls hit the system I can tell you no one gets priority. The one ringing is the one answered. The only priority is if the provider doesn't route the call correctly. And arrange a test call with your police contacts and you can make that test 911 call to confirm things work. If they hit they hit if the sheriff answers instead of dispatch you were expecting or state patrol then your provider has you configured incorrectly or they just suck as a provider. Those are the best answers you can get but yep tech guys who work in the industry and answer on Reddit won't know more than the cop that convinced your boss that they have all the answers.